Accessory structures of the eye

Structure of the eyelid: anterior and posterior surfaces

Histologically, the eyelid is a layered structure clearly divided into an anterior cutaneous part, a posterior conjunctival part, and a supportive musculoglandular framework.

The anterior surface (cutaneous part) is formed by thin skin consisting of the classic layers: epidermis and dermis. Numerous cutaneous appendages are located here:

• Fine vellus hairs grow over the main surface, while large hairs—eyelashes—are located along the free eyelid margin.
• Sebaceous glands, characterized by a holocrine mode of secretion, are located at the roots of the eyelashes.
• Sweat glands are represented by two varieties: apocrine glands are located immediately adjacent to the eyelashes, while merocrine glands are evenly distributed throughout the rest of the eyelid skin surface.

The posterior surface (conjunctival part) is tightly lined with a mucous membrane. It covers the inner aspect of the eyelid, smoothly reflects onto the eyeball at the fornix, and this epithelial sheet continues directly into the corneal epithelium. Histologically, the posterior surface consists of two components:

1. Epithelium: stratified squamous non-keratinized.
2. Lamina propria: formed by loose connective tissue (LCT). Its most critical hallmark is extremely rich vascularization with a dense capillary network.

Internal eyelid framework and muscular apparatus

The internal part of the eyelid performs supportive and motor functions, and it also contains specific glandular structures.

The framework is based on the tarsal plate (so-called tarsal cartilage). Despite its historical name, it consists of dense connective tissue and is merely a histological cartilage vestige. Embedded within this plate, perpendicular to the eyelid margin, lie the Meibomian glands (approximately 30 on average).

Morphologically, Meibomian glands are modified sebaceous glands. They are classified as simple branched tubuloalveolar glands with a holocrine mode of secretion. The excretory ducts of these glands open at the eyelid margin posterior to the lash line.

Between the skin and the tarsal plate lies the muscular apparatus, formed by bundles of skeletal muscle tissue:

• Levator palpebrae superioris muscle: its fibers are oriented strictly vertically (present in the upper eyelid).
• Orbicularis oculi muscle: represented by circular muscle bundles that run near the free margin of both eyelids.

In addition, small accessory lacrimal glands are concealed beneath the conjunctiva in the region of the eyelid fornix, contributing to the continuous lubrication of the eye.

Lacrimal Apparatus (Apparatus lacrimalis)

The lacrimal apparatus includes structures responsible for the abundant secretion of tears and the pathways for their continuous drainage.

The main lacrimal gland (Glandula lacrimalis) is located in the superolateral aspect of the orbit. Morphologically, it is a compound tubuloalveolar gland. It actively produces lacrimal fluid, the chemical composition of which includes 1.5% sodium chloride, 0.5% albumin, and a small amount of mucus.

Lacrimal pathways (drainage pathways) ensure secretion evacuation. The drainage system originates at the medial angle of the eye and consists of a strictly defined sequence of anatomical structures:

1. Lacrimal canaliculi (superior and inferior).
2. Lacrimal sac.
3. Nasolacrimal duct, which opens into the inferior nasal meatus, draining fluid into the nasal cavity.

Histologically, the walls of the lacrimal sac and nasolacrimal duct are lined with bistratified and pseudostratified columnar epithelium, reflecting their fluid-conducting function.

Class and exam questions

What else to study on this topic

• Cellular architecture of simple branched tubuloalveolar glands using Meibomian glands as an example.
• Structural features of the conjunctival lamina propria and its capillary network.
• Histological differences between apocrine and merocrine sweat glands of the eyelid skin.
• Morphology of the compound tubuloalveolar lacrimal gland and stages of secretion.
• Topography and functions of the small accessory lacrimal glands of the conjunctival fornix.

Other topics in "Sensory organs: organs of vision and olfaction"

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.